The row of houses above Beaver Grade Road in Moon Township appears to be perched on a sturdy hillside of chunky gray rock with a bit of green grass peeking out near the top. But last January, melting snow and heavy rains saturated the soil, penetrating deep into the clay stone layers common in southwestern Pennsylvania, called red beds. The hill began to slide.
Red beds and water are a slippery combination.
“If you’ve ever stepped on wet clay, you might fall down because it’s so slippery,” said Shane Szalankiewicz, district bridge engineer for PennDOT who oversees its geotechnical unit. “Underneath some of these areas, you have underlying bedrock or poor soil conditions that can be triggered by water.”
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PennDOT closed the road and started drilling to get a good look at the soils. It found the slick clay stone and decided on a landslide fix called a rock buttress. It piled 52,000 tons of rock onto the slope and pinned it in place to stabilize it.
Two of those houses in Moon Township were too close to the slide, and the homeowners had to be temporarily evacuated.
They aren’t the only residents with risk in their backyard. A colorful map depicting Allegheny County’s past, current and potential landslide landscape shows only a few sturdy plots.
“[With landslides] most often, it’s just this topography,” Szalankiewicz said. “It’s the soils that you’re dealing with. It’s the bedrock that you’re dealing with, those are the factors — and water.”
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And Pittsburgh is primed to get wetter. The region’s biggest climate threat in the coming decades is more rainfall, particularly more extreme storms, according to the Pennsylvania Department of Environmental Protection’s Climate Impact Assessment. These storms strain local infrastructure and lead to more landslides and flooding — already familiar threats to house foundations across the region.
Today’s houses sit on past development decisions such as urban sprawl, zoning that permitted building on steep slopes, and a lack of coordination between the region’s fragmented municipalities.
This soaked future hasn’t gone unnoticed by local and state government agencies and nonprofits. From a multiyear master plan to understand and update Pittsburgh’s aging stormwater infrastructure to a bill lingering in the state legislature creating a landslide insurance program for homeowners, efforts are underway to stem climate impacts.
But predicting extreme weather events and their fallout is still a guessing game — making it difficult for local leaders and residents to make plans, budget and batten down.
“The issue with these extreme storms is you can’t predict when, where,” said Daniel Bain, associate professor of geology and environmental science at the University of Pittsburgh. “We don’t exactly have a clear idea which ones are going to be the most important, most frequent, most damaging.”
A legacy for a different future
Climate models project that the average amount of rain will increase in southwestern Pennsylvania in the coming decades. And so will the frequency of heavy storms, suggesting the region is likely to experience heavy downpours scattered between periods of drought, according to the DEP Climate Assessment.
In fact, the rise in intense rain events has already arrived. More than 80% of sites in Pennsylvania surveyed by the state climatologist had more heavy rain events in the 2010s than during the 1980s, according to the DEP. In April, Pittsburgh got drenched with double its average rainfall during a few days of intense rain, triggering flash floods, landslides and a submerged fountain at Point State Park.
“The highest-risk climate hazard that faces Pennsylvania is flooding and flooding due to localized events whereby we wind up with high-intensity rainfalls. We wind up with flash flood events,” said David Althoff, director of the energy programs office at the DEP. “A lot of that would be due to topography and the way that stormwater is conveyed, through small streams and creeks. But also through built stormwater infrastructure, which frankly, gets overwhelmed quickly because it may have been built 50, 60, 70 years ago and really didn’t contemplate this increase in both annual precipitation and also of sudden flash flood events.”
The confluence of intense rainfall events, steep topography, aging infrastructure and housing built for a different climate future threaten homes around the region. While few communities in Allegheny County can guarantee a dry basement, many of the spots in the county that have the greatest risk of flooding are also the places that have suffered historic disinvestment. Places such as McKeesport, Duquesne and Sharpsburg, and Pittsburgh neighborhoods, such as Hays, sit on the 100-year floodplain.
Pittsburgh’s past development haunts future flooding risk. A history of mining affects how water can flow. Former steel mills and industrial plants leave behind contaminated soils.
“Historically contaminated soil also moves whenever water flows through the soil or there’s runoff or erosion,” said Heather Manzo, executive director of the Allegheny County Conservation District. “That can spread contamination.”
Pittsburgh’s history of urban sprawl and historic zoning codes emphasize single-family homes in the city of Pittsburgh and suburban Allegheny County. Developers built more concrete environments and left less green space to absorb excess water. So when it rains — instead of soaking into the ground — water flows straight into the public water and sewer systems.
“We have a lot of low-density housing, low-density development that creates more impervious surfaces on our ground,” said Kyla Prendergast, senior environmental planner at the city of Pittsburgh. “The more individual homes that are required means that individuals are taking up more land space.”
Past Pittsburghers built homes on the city’s steep slopes and in its deep valleys. Houses in spots like Hays or neighborhoods with the word “Run” tacked onto the end lie “in a bowl area, which I didn’t used to think was possible until I moved here, that you could actually live in an area where people say it’s uphill both ways,” said Tony Igwe, senior group manager of stormwater at Pittsburgh Water and Sewer Authority. Water flows downhill from the roadways and pools at the bottom, often flooding basements.
Landslide remediation on William Street in Mount Washington in 2024. Photo: Katie Blackley / 90.5 WESA
Homes of former steelworkers pepper the sharp hillsides that tumbled from time to time with heavy rain. Work began in May to fix a landslide on William Street on Mount Washington that threatened homes. It’s part of a $10 million project funded mostly by Federal Emergency Management Agency dollars. It isn’t the first time the street slouched. Photos from the University of Pittsburgh Archives show a past landslide on William Street from 1933.
There was little to keep developers from developing even on unstable soil for most of Pittsburgh’s history. The only “notable effort,” according to Joel Tarr, professor emeritus of history and policy at Carnegie Mellon University, came in 2005. Pittsburgh’s city council passed a series of ordinances to protect certain landslide-prone areas while also allowing development that met certain hillside-responsible standards.
“There’s always controversy about land use,” Tarr said. “What can be developed and what can’t. Land is valuable. And some people, some organizations are very interested in driving and pushing to develop land, and willing to ignore landslide possibilities because the city was in desperate need of finances.”
Subterranean antagonist
Houses inherit past land-use decisions. In Pittsburgh, it’s a complicated inheritance that includes an underground stormwater history. Most development in Allegheny County is done on land that had previously been developed, according to Manzo.
Each house affects the stormwater impact of the community. Stormwater knows no boundary lines. The region is a patchwork of municipalities, each with their own codes and underground systems, both stormwater and combined sewage. Fitting the pipes between all of these agencies and individual property owners is a twisted challenge.
When new buildings, shopping centers and housing are built, they must comply with new zoning codes, including updated stormwater regulation and controls for a higher volume of water. But older developments might not have any stormwater controls or systems that haven’t been maintained.
“There’s a lot of development around the county that doesn’t have any stormwater control,” Manzo said. “And so that’s producing all kinds of fast stormwater outflow whenever there’s a rain event. And where does it go? It goes straight to the infrastructure. That’s why it’s just so critically important that things are designed to handle the current reality and that the infrastructure is able to handle it. Otherwise, that water is going to go into a stream, a creek, a roadway, a backyard, a basement. And so, that legacy housing is creating stormwater that could impact the houses or the community around it.”
Every development project in the county must now show how the stormwater will be managed. But regulations vary among municipalities. The state DEP is in the process of updating its stormwater regulation to account for a higher volume and flow of rainfall and give new guidelines to developers to make sure that the stormwater they’re controlling through new housing sites is set to a new standard. They were last published in 2006.
The city of Pittsburgh already updated its stormwater code in 2022 to account for increased rainfall due to climate change. And in its zoning code, it has something called rainwater performance points, which allow developers to increase the height at their property if they’re able to manage more stormwater.
“It’s really effective at making sure that we’re getting denser housing, as well as managing more stormwater,” Prendergast said.
Fourth Avenue near Wood Street in Downtown Pittsburgh after the St. Patrick’s Day flood of 1936. Walter J. Teskey / University of Pittsburgh
But there isn’t a singular stormwater system in the city of Pittsburgh. There’s a stormwater-dedicated sewer system in about 25% of the city and a combined sewer network that carries both sanitary and stormwater for the rest of Pittsburgh, which is managed by PWSA. The 83 municipalities throughout Allegheny County each have their own system and their wastewater flows to Allegheny County Sanitary Authority for treatment.
As little as one-tenth of an inch of rain can overwhelm the old sewer lines and send sewage into Pittsburgh’s three rivers. ALCOSAN is under a federal consent decree that requires it to fix the sewage overflow problem. ALCOSAN’s $2 billion solution includes building large underground storage tunnels to expand the system and prevent around 7 billion tons of sewage from streaming into the rivers during heavy rains each year.
But ultimately, Igwe said, preventing flooded basements and controlling stormwater is a land-use issue. “When we have better development, ordinance and code for managing stormwater on-site or managing stormwater within the development, that helps us. Even the right of ways that the city owns is less than 10% of the whole entire city. So, 90% of the stormwater that falls on the city is falling on private property.”
At this point, PWSA says fixing the entire sewer network would be too much for a ratepayer’s pocket.
“In order to make that something that is across the city where you never flood, you couldn’t afford the sewer rates that would result,” Igwe said.
“Our city wasn’t really built with the water in mind,” Prendergast said. “It was just sort of built to kind of plug it into the sewer systems. But at some point with our increasing rainfall, the sewer systems are reaching a capacity. And it’s either we put bigger pipes in the ground or we manage more water at the surface.”
Shoring up for future storms
Allegheny County’s fragmented government is another challenge. Some 130 municipalities reside in the county, many operating with a small staff, public authorities and private water companies.
“The thing that is really important is that municipalities work together with their neighbor,” said Eric Raabe, director of infrastructure and resilience at CONNECT, a nonprofit focused on local governance in southwestern Pennsylvania. “A developer might work with two different municipalities because the water that is created by the development doesn’t just stay where the development is, it flows downhill. And another municipality might be downhill. In so doing, one of the biggest, most high impacts would be green infrastructure that is multi-municipal in scale.”
From living green roofs atop buildings at the University of Pittsburgh to Tree Pittsburgh’s $8 million effort to increase Allegheny County’s tree canopy, green infrastructure is popping up around Pittsburgh. These projects absorb water by mimicking nature.
Researchers at RAND, the public policy think tank, are studying wet basements in Homewood and the Hill District, which are prone to flooding from the Negley Run watershed, to get a sense of the scope of the issue and how it impacts the health of residents. Generally, mold growth can trigger and exacerbate asthma for people. Black mold can cause headaches and impact the central nervous system, according to the EPA. But the EPA hasn’t established thresholds about how much mold or what types of mold are especially unhealthy, according to Linnea Warren May, associate policy researcher at RAND: “It’s likely that most people have mold in their basements, but we don’t know exactly how much or how much is especially bad.”
For housing in chronic flooding areas or hillsides prone to severe slides, relocation is the last resort. The city of Pittsburgh is working with the Federal Emergency Management Agency and the Pennsylvania Emergency Management Agency on a process to determine where and how to buyout homeowners who live in risky areas and want to move. One possible option is to buy the rights to properties and turn these dangerous areas into conservation easements or greenways.
Expanding the greenways program could be included in the city’s upcoming comprehensive planning process. Updating the zoning code to include development regulations around landslide and flood-prone areas is also being considered. One potential mechanism is for the city to swap the development rights of a piece of property in an environmentally sensitive area for another piece of land in a safer area.
“We’re trying to figure out how to allow the government to regulate land use without taking the rights away of residents and private property owners to use their land,” Prendergast said. “It is a difficult, delicate line, and it is usually evolving. But going forward, we’re really trying to use different tools.”
To stabilize Pittsburgh’s future development, Pittsburghers must learn from past slips, according to Tarr.
“A lot of the housing in the city was built a long time ago. And you look and say, ‘Well, how could they have built along the edge of that ridge? How could they build at that site?’” he said. “You thought they would have been more aware of the possibility of risk, but of course, they were willing to ignore it. And everybody says, ‘Well, it’s raining heavily this year, but it won’t do it again for a long time.”
